The present invention relates to a wire for use in a so-called wire harness provided within an electrical equipment and an ordinary transport machine, and more particularly to a wire for a low-pressure press-connecting terminal which wire can be suitably press-connected to a connection portion of the metal terminal.
Further, the present invention relates to a compressed stranded conductor used as a wire, and more particularly to a multi-layer compressed concentric twisted (stranded) conductor having a space factor of not less than 99%, and also to a method of producing the same.
Generally, in an electrical equipment and a transport machine such as an automobile, a wire bundle, commonly referred to as a wire harness consisting of a predetermined number of wires pre-assembled together into a required length in accordance with the arrangement of electrical component parts within the electrical equipment or the machine, is used for the convenience of assembling of such device. In such a harness wire and particularly one of a low-pressure design, a press-connection, by which the connection is completed without the need for peeling an insulative covering material, is often carried out. In this press-connection as shown in FIG. 13, a harness wire w is forced into a slot t.sub.1 of a press-connecting terminal t, so that a covering material w.sub.1 is torn by an edge of the slot t.sub.1, thereby making electrical connection between a conductor w.sub.2 in the wire and the edge of the slot t.sub.1 (See Examined Japanese Patent Publication No. 4-12593).
FIGS. 14, 15 and 16 show conductors of a wire heretofore used within an electrical equipment and a transport machine such as an automobile.
The assembled-type twisted wire A.sub.6 of FIG. 14 is formed by twisting a number of constituent elements a together, and the position of the constituent wire elements a with respect to one another is not constant, and is unstable. For example, when examining this positional relation in the direction of its length, those element wires, disposed near to the center of the wire in the beginning, is often located at an outer layer at another portion of the twisted wire.
FIG. 15 shows a most common 7-core twisted wire A.sub.7 having 6 wire elements a twisted around a central element a.
FIG. 16 shows a multi-layer concentric twisted wire A.sub.8 in which a first layer L.sub.1 of 6 wire elements a are twisted around a central wire element a, and a second layer L.sub.2 of wire elements a and a third layer L.sub.3 of wire elements a are sequentially twisted around the first layer. In this case, the directions of twisting of the constituent wire elements of the first layer L.sub.1, the second layer L.sub.2, the third layer L.sub.3 . . . are alternately opposite, and these layers are different in twisting pitch from one another.
Such a multi-layer alternately twisted wire for use as an electric wire is subjected to compression for the purpose of enhancing the ability of press-connection to a connector, reducing the electric wire into a small diameter, saving the amount of the insulative material, enhancing a stress corrosion cracking resistance, and enhancing electrical characteristics, and various methods for this purpose have been proposed.
The condition of press-connection of such a conventional wire is shown in FIG. 17. When a harness wire w' is further forced into a slot t.sub.1 ' of a press-connecting terminal t', with its insulative material w.sub.1 ' cut by an edge t.sub.2 ' of this slot (see FIG. 17(A)), a stranded conductor w.sub.2 ' twisted into a substantially circular cross-sectional shape gets out of shape (see FIG. 17(B)). Therefore, electrical connection between the stranded conductor w.sub.2 ' and the edge t.sub.2 ' becomes unstable.
In order to overcome such an unstable electrical connection, recently, strands have been integrally joined together collectively by plating, or a compressed conductor or the like as shown in FIG. 18 has been used, thereby enhancing the reliability in press-connection. FIG. 18 shows the compressed conductor A.sub.9 ' compressed at a rate of about 93%. Here, the compression ratio means a space factor, and means the ratio of the actual cross-sectional area of the stranded conductor to the area of a circle circumscribing the cross-section of the stranded conductor. These constructions are both directed mainly to the ability of maintaining the shape, and sacrifice an expected flexibility of the stranded wire accordingly. Furthermore, since spaces s are formed between the constituent wire elements a', the small-diameter design has not yet been sufficiently achieved.
Referring to wires at large, examples of multi-layer wire subjected to compression are shown in FIGS. 19 and 20.
FIG. 19 shows the condition of compression of constituent wire elements a' of a stranded conductor A.sub.10 ' compressed at a time as in the multi-layer wire (FIG. 16) having the layers twisted alternately in opposite directions. The directions of twisting of the layers are alternately opposite, and the constituent wire elements a' necessarily intersect the wire elements of the upper and lower adjacent layers, and therefore the degree of compression is naturally limited.
FIG. 20 shows a compressed conductor A.sub.11 ' in which compression for shaping purposes is effected for each layer. However, this type is limited to the use as a high-voltage wire, and besides the purpose of compression is to enhance electrical characteristics rather than to achieve a small-diameter design. Furthermore, the direction of twisting of constituent wire elements a of the layer L.sub.1 ' is opposite to the direction of twisting of constituent wire elements a of the adjacent layer L.sub.2 ', and the layers are different in pitch from each other.
With respect to methods of producing these conventional wires, there have heretofore been used various kinds of twisting machines for producing a wire having a relatively small space factor (which means that the wire is not subjected to a high degree of compression), and the wire can be produced, using any of these machines. In this case, they are classified into the type in which pre-shaped constituent wire elements in the form of profiles are twisted together and the type in which wire elements are twisted together, and then are passed through a die or the like to be compressed.
In the production of a wire with a relative large space factor in which wire elements are compressed at each layer into a circular cross-sectional shape (see FIG. 20), a first layer L.sub.l ' of wire elements are compressed around a central wire element a while being twisted, and further a second layer L.sub.2 ' of wire elements are compressed around the thus shaped twisted wires L.sub.1 ' while being twisted. Therefore, in order to produce the compressed conductor A.sub.11 ' of this construction, there are required machines for applying compression which are equal in number to the layers of twisted wires. Therefore, the apparatus inevitably has a large overall size, and is complicated.
Another method of producing a twisted wire having a higher space factor employs a wire twisting and drawing machine disclosed, for example, in Unexamined Japanese Patent Publication No. 1-95420. With this method, however, in so far as a wire of an ordinary construction is used as a base wire, only the twisted wires of up to the first layer can be compressed at a high degree.
Thus, in the wires for a press-connecting terminal in an electrical equipment and a transport machine, various attempts have been made, directing attention to the ability of maintaining the cross-sectional shape of the conductor in order to enhance the reliability of the press-connection; however, because of such an improved ability of maintaining the cross-sectional shape, the flexibility has been sacrificed, and as a result there have been encountered problems such as (1) a lowered operation efficiency in connection with the wiring and (2) a degraded fatigue resistance to vibrations during use. Furthermore, because of a demand for a small-size design of various devices, (3) the wire for a harness has been required to have a small diameter.
Therefore, it can be said that requirements for the wire for a press-connecting terminal within an electrical equipment and an ordinary transport machine are (A) to have a sufficient flexibility, (B) to have a sufficient ability to maintain the cross-sectional shape, and (C) to have a sufficiently small diameter. However, there has been no prior art construction which meets all of these requirements.
For example, in the twisted wire subjected to collective plating, the cost is clearly increased by the plating step. The assembled-type twisted wire, as well as the concentric twisted wire providing round wire elements, does not have the ability of maintaining the cross-sectional shape, and also can not have a small-diameter design. In the 7-core compressed stranded conductor, the flexibility, the ability of maintaining the cross-sectional shape and the small-diameter design have been achieved to a considerable level, but have not yet been sufficient.
Referring to power wires or cables at large, some of them have such a configuration that they can be compressed, but even if they are reduced in size so as to be used as a wire for a press-connecting terminal of an electrical equipment and a transport machine, sufficient compression can not be expected, or a production method inevitably becomes complicated, and is large in size, which increases the cost.